1. Field of the Invention
The present invention relates to a heat-fixing belt in a device utilizing electrophotography, such as a copier or a printer. The present invention also relates to a manufacturing method thereof, and an electromagnetic heat-fixing device using the same.
2. Description of the Related Art
In an image forming device such as a copier or a printer employing an electrophotographic system, the process of fixing a toner image formed on a recording material such as paper to make a permanent image has been conventionally called a “fixing process”. Conventional fixing processes include methods of press fixing, oven fixing, and solvent fixing, however, the thermal press fixing method has been most commonly used. This is due to the fact that the thermal press fixing method can effectively transmit heat and fix the toner image more firmly than other methods, and furthermore, it is comparatively safe.
The thermal press fixing method is a method in which a recording material having an unfixed toner image formed thereon is passed through a nip formed by two heated rolls or belts. The unfixed toner, which is heated by the rolls or belts and brought into a fused state when passed through the nip, is pressed onto the recording material and fixed thereto by the nip pressure.
The roll or the belt of a fixing member has a releasing layer provided on its surface, so as to have good separability and to prevent the surface from being fixed to the fused toner. Further, the roll or the belt is heated by a heating member in order to transmit heat to the toner image.
A method of heating the roll or the belt from inside the roll with the radiant heat of a halogen heater, which is provided in the roll, has been conventionally used. With this method, it takes much time to heat the surface of the roll to be heated to the point where the toner image can be fixed, because the roll is heated from the inside. For this reason, when a user copies or prints something, it is necessary to wait for the printed item. Moreover, in order to make the waiting time as short as possible, the surface of the fixing roll is continuously heated at a high temperature during standby so as to maintain a temperature that is lower than the fixing temperature. However, this method increases power consumption due to the standby heating, hence, the method does not satisfy the recent demand to provide energy-efficient machines.
Thus, a fixing device using a thin film and a fixed heater is taught utilizing an energy-saving fixing method in certain patent documents such as Japanese Patent Application Laid-Open (JP-A) Nos. 63-313182 and 4-44074. There has also been a widely used method of using a thin film belt as the fixing member, and heating the fixing member with a planar-resistant heating body arranged in the thin belt. In this method, as compared with the method of heating the roll from within the center, it is possible to shorten the fixing time because the belt can be heated without a heat insulation air layer, and further, the method does not require heating the center of the roll.
However, in the method using the above-mentioned belt and planar-resistant heating body, the planar-resistant heating body itself possesses a heat capacity, and it is difficult to shorten the time necessary to reach a fixing temperature to the point that the user does not feel a waiting time. It is also difficult to make the temperature distribution of the planar-resistant heating body uniform in the axial direction. Therefore, considering the current state of the above-mentioned method, sufficient energy conservation and high-quality image forming have yet to be achieved.
Meanwhile, a method of heating a fixing member with an induction heating system has been studied in recent years (e.g., JP-A Nos. 11-352804, 2000-188177). The heating principle of an electromagnetic induction heat-fixing system will be explained below.
The electromagnetic induction heat-fixing system requires not only a heat-fixing member and a press member, which are conventionally used, but also a coil and a high-frequency power source. The coil is arranged at a position inside the heat-fixing member or outside and near the heat-fixing member, and is electrically connected to the high-frequency power source. A metal heating layer in either the shape of a roll or a belt can be used as the heat-fixing member, which is heated by electromagnetic induction.
A high-frequency alternating current is passed through the coil from the high-frequency power source. At this time, magnetic flux is generated in the coil in a direction perpendicular to a plane wound by the coil corresponding to the direction of the current. The magnetic flux crosses the metal heating layer of the heat-fixing member arranged near the coil, generating an eddy current that in turn generates a magnetic field in a direction canceling this magnetic flux generated in the metal heating layer. Since the resistance of the metal heating layer is determined by the type of metal and the thickness thereof, the electric energy of the generated eddy current is converted to thermal energy. A fixing device using heat generated in this manner is referred to as an electromagnetic induction heat-fixing device.
In this method, the surface of the member to be heated can be heated effectively and thermally efficiently, making it possible to shorten the time necessary to reach a fixing temperature to an absolute minimum. As described above, the induction heat-fixing device includes a roll-type device and a belt-type device. In both types, by running a high-frequency current through the coil arranged near the member to be heated, an induced electromotive force is generated in the metal heating layer of the member, creating the eddy current that heats the member. In the roll-type device, a core metal can comprise the heating layer and be heated to a fixing temperature if an appropriate material is selected. The core metal material should be of a thickness capable of generating the eddy current with the coil, and heating the member with the eddy current. However, in the case of a roll-type device, it is the core metal that is heated, so the fixing temperature can be reached in a shorter time. This is because unlike conventional heating systems, there is no air layer, however, the core metal needs to have a thickness of several milli meters because it must possess rigidity. As a result, the core metal of the beating layer inevitably has a large heat capacity, which in turn increases the time it takes to heat the core metal. Accordingly, it is impossible to sufficiently shorten the time it takes to reach the fixing temperature.
Methods of forming a belt-type induction beat-fixing member include a method of using the metal heating layer as a substrate, and a method of forming a metal heating layer on a heat-resistant resin substrate. In the case of a belt using a metal heating layer as the substrate, the thickness of the substrate of the metal heating layer needs to be dozens μm to 200 μm thick because the substrate needs to be strong to a certain extent. This increases the heat capacity of the substrate, which increases the amount of time necessary to heat the surface of the belt, though not to the same extent as the roll-type device. Further, in order to form a nip with a press member and the belt, it is necessary to arrange a pressure applying member at a position opposite to the belt inside the belt. In many cases, a rubber pad is used as this pressure applying member because it forms the nip with the press member at a uniform pressure and ensures a nip width, however, this pad does not slide well against the metal substrate and is thus prone to intense deterioration.
Meanwhile, in the case of a belt using a substrate made of heat-resistant resin, engineering plastic having a heat resistance of 200° C. or more and having sufficient strength, such as polyimide or polyamide imide, is used. In this case, because the resin substrate ensures strength, the metal heating layer can be thinned as long as it can generate a sufficient amount of heat. Thus, in comparison with a belt having a metal substrate, it is possible to shorten the time it takes to reach the fixing temperature. Moreover, since the substrate is resin, it slides well against the pad inside the belt forming the nip.
The metal heating layer needs to be formed on the substrate in a uniform thickness. In certain cases, depending on the type of metal, the thickness of the layer can be decreased if the metal has low resistance, hence, it is possible to reduce the time it takes to reach the necessary fixing temperature. Generally speaking, metals such as copper, aluminum, and nickel are often used for the metal in the heating layer. Using these metals, a thin metal film can be formed on the heat-resistant resin with methods such as plating, vapor disposition, and sputtering. As described above, there is an optimum thickness, depending on the type of metal used, and the thinner the thickness, the less rigid the belt itself becomes. A thinner belt is more flexible, making it easier to form a suitable nip, thereby forming a fixed image of better quality. In addition, the heat capacity of a metal heating layer with a thinner film can be decreased, providing the advantage of shortening the time required to reach the necessary fixing temperature. It is therefore necessary to select a metal that has low resistance and that can heat despite being thin, and to form the metal film as thinly and uniformly as possible.
However, in the current state of art, there are certain problems with the fixing belt of the electromagnetic induction heating system in which the above-mentioned thin metal film layer is formed on the resin substrate. These problems relate to (1) the durability of the thin metal film layer and (2) the adhesiveness of the thin metal film layer to the resin substrate.
(1) Durability of the Thin Metal Film Layer
The thinner the film of the metal heating layer is, the less the heat capacity becomes, hence, the time required for the metal heating layer to reach the fixing temperature becomes shorter. Furthermore, the belt itself becomes more flexible which in turn improves the image quality, however, the strength of the metal heating layer decreases. Since the object is to use the belt for the induction heat-fixing device in order to fuse the toner on the recording material while applying pressure to the toner to firmly fix the toner to the recording material, the induction heat-fixing belt is used such that a nip load is pressed between the induction heat-fixing belt and the press members (e.g., press roll, press pad, press belt and the like) arranged at a position opposite to the induction heat-fixing belt. At this point, if the metal heating layer is thin, in some cases, the nip load necessary for fixing causes defects such as cracks or splits. Moreover, even when the nip load is low, the heating layer is passed through the nip many times causing repeated bending stress, and defects can occur in the metal heating layer such as cracks or splits.
When such defects are caused in the metal heating layer of the electromagnetic induction type heat-fixing member, the resistance of the heating layer is increased or the inside of the metal heating layer becomes electrically insulated, thus decreasing its heating capability. When the cracks do not become splits but rather groove-shaped defects, the thickness in those regions thins, which in turn causes abnormal in the same regions. This abnormal heating burns or fuses the separating layer coated on the surface, which drastically deteriorates the durability of the part.
Thus, as disclosed in JP-A No. 2001-341231, a proposition was made in which the substrate is endowed with flexibility to thereby reduce the mechanical stress applied to the metal heating layer by regulating the imidization rate of the polyimide resin layer.
However, depending on the stress, the proposition disclosed in JP-A No. 2001-341231 does not always eliminate the mechanical stress applied to the metal heating layer in the nip. In other words, simply making the substrate flexible does not sufficiently prevent the formation of cracks.
(2) Adhesiveness of the Metal Heating Layer to the Resin Substrate
Known methods of manufacturing a film-shaped member made by laminating a thin metal film on a heat-resistant resin layer include a method of bonding a heat-resistant resin film to a metal foil with an adhesive, and a method of forming a thin metal film on a heat-resistant resin film by chemical or physical plating.
However, the adhesion in the above-mentioned method of bonding a heat-resistant resin film to a metal foil with an adhesive is not reliable when the thin metal film is repeatedly heated with electromagnetic induction. Even in the method of forming a thin metal film on a heat-resistant resin, it is generally difficult to make the heat-resistant resin layer such as a polyimide or aromatic polyamide (aramid) firmly adhere to a thin metal film made from copper or the like.
In order to improve adhesion, JP-A No. 5-299820 proposes a technology where a metal vapor deposition film is formed on a polyimide, after which sequential lamination of a copper layer by electron beam heating vapor deposition and another copper layer by electrolytic plating is performed on the metal vapor deposition film.
Further, JP-A No. 6-316768 discloses a technology in which fluorine is included in the polyimide and then, in order to make this fluorine an adhesive site, the polyimide is first subjected to first etching by use of an aqueous solution containing hydrazine. Next, it is subjected to second eching with naphthalene-1-sodium, thereby making the copper adhere easily to the polyimide.
Still further, JP-A No. 7-216225 discloses a technology for enhancing the adhesiveness of a thin metal film to a polyimide by mixing powdered metal into a polyimide precursor.
Meanwhile, even in the case where the heat-resistant resin is an aromatic polyimide (aramid), JP-A No. 6-256960 proposes a technology that subjects aromatic polyimide to etching with an aqueous solution containing hydrazine and alkaline metal hydroxide, and then catalyzing to obtain non-electrolytic plating.
However, as described above, depending on the stress at the nip, the mechanical stress in the metal heating layer is not always eliminated by making the fixing member flexible, hence, the prevention of cracks cannot be sufficiently accomplished simply by making the fixing member flexible.
Still further, in order to obtain stable adhesion in the case where the heating layer is formed on the resin substrate of an insulator, the technologies disclosed in the above-mentioned patent publications do not provide sufficient adhesion and further, they require a complex manufacturing processes, which inevitably increases manufacturing costs.
Therefore, a fixing belt capable of more effectively and compatibly preventing cracks from mechanical stress and shortening of the warm-up time is necessary, as well as a manufacturing method thereof. Further, it is necessary to provide an electromagnetic induction heat-fixing device capable of preventing the deterioration of its heating capabilities, and of maintaining high image quality for a long time.
Still further, it is necessary to provide a fixing belt that improves adhesion to the metal heating layer and enhances durability, and to provide a manufacturing method thereof.